The present invention relates to a high-speed or high-definition dot matrix printing head whereby the print pins are guided by the matrix through two parallel, side by side rows of holes located on the matrix and lying in a plane parallel to the printing plane.
Known dot matrix printing heads, particulary 24-pin types, for high-speed, low-definition (draft) printing, or high-definition NLQ (near letter quality) printing at slower yet still considerably fast speed, present a printing pin guide matrix with two rows of holes arranged side by side, so that the tips of the pins striking the ribbon for imprinting an ink dot on the printing paper are arranged in two parallel, side by side rows, usually perpendicular to the operating plane of the printing head, hereinafter referred to as the "printing plane". The print characters are formed by printing a number of dots on the sheet according to predetermined grids (or matrixes), by selectively activating the printing pins, against the action of elastic means and via rocking striker elements controlled by electromagnets, while at the same time shifting the printing head accordingly in the printing plane. According to a first known embodiment, the matrix supporting and guiding the printing pins is formed in two parts designed to slide perpendicular to the printing plane so that, for high-speed, low-definition printing, the pins in both rows (each supported on one of the component parts of the matrix) are arranged side by side in the same plane. For high-definition printing at slower speed, on the other hand, the two parts of the matrix are shifted vertically so as to offset one row of pins in relation to the other and so print characters with partially overlapping dots. In view of the severe operating stress involved, printing heads featuring a two-part matrix of the aforementioned type are of fairly complex, intricate design and, therefore, poorly dependable.
A second known embodiment features a one-piece matrix with offset pin guiding holes, so that the pins are normally arranged in two offset rows, as required for high-definition printing. For high-speed printing, a special mechanism provides for turning the entire printing head through a given angle, so as to set both rows of pins obliquely in relation to the printing plane, with the pins in both rows lying in the same plane. In addition to being equally complex in design and requiring said rotation mechanism, a major drawback of the head according to the above embodiment is that it does not allow for switching from high-speed to high-definition printing and vice versa while it is operating, due to the inertia of the same preventing it from being rotated accurately while moving. Heads of the above type must therefore be set to a given print mode while stationary, and stopped for switching to the other mode.